where would you least expect to find an ionization nebula?

A sedimentary origin for chondritic meteorites

Anne M. Hofmeister , Robert Eastward. Criss , in Heat Send and Energetics of the Earth and Rocky Planets, 2020

eleven.4.3 Processing in the winds of accretion

Densifying a nebula is essential to planet and star formation (Fig. xi.6D–F). In Chapter 4, we proposed that these bodies offset existed as iron protocores. Magnetic forces, friction welding, and cold welding supplement gravitational attraction, and more importantly tin hold the iron particles together after a standoff. The ductility of iron also promoted initial growth, whereas silicate grains would bounce and/or fracture rather than adhere during grain-to-grain collisions. Once a metal protocore forms, breakable silicate grains would even so fracture on impact, but would be held on by gravity. Chapters iv and nine Chapter four Affiliate ix talk over germination of early bodies further. The present chapter is concerned with how chondrules and chondrites would grade during planetary growth, which too drew in the iron remaining in the nebula with the mineral grit, after the protocore formed.

Every bit the incipient planets drew in nebular affair, strong circling winds are expected (Fig. eleven.6E). The winds carry mineral grains toward them, in a stream (Fig. xi.6I), and so collisions and abrasions exist, but these are not impacts at high speed because all grains in any given region take the same speed and management. Heat evolved is small-scale and speedily dissipated by radiation. The chief process is rounding of grains in the winds of accession, with some adhesion and growth. Chemic reactions announced to be substantial during this stage (Table xi.3) because oxidized ordinary chondrites are rather one-time (Table 9.4). This stage can be envisioned as a gathering cyclonic sandstorm of huge proportions. The speed may be quite slow, particularly at outset, when a small protocore is the center of gravitational allure. But, every bit the protoplanet grows, it pulls more than strongly on its surroundings, and then the forces will increase. We advise that in the inflows, upwardly to the point of touch, the master process is abrading large mineral grains and aggregating modest nebular grains into chondrules.

Tabular array 11.three. Processes perhaps affecting formation of meteorite components ^a .

Grain formation in ejecta	Grain growth in repose clouds	Tumbling during accretion
Aging stars loose mass b	Common cold welding	Vertical collapse of nebula
Ions and atoms ejected	Collisions/aggregation	Cyclonic winds during spin up
Cooling of hot gas	CO-H2 gas reactions, etc.	Chafe/rounding
Nucleation	Icing of grains	Frictional heating/welding
Electrostatic interactions	Catalytic reactions	Coatings (rims) grade?
Chemical vapor degradation	Graphitization	Lightening strikes
Drifting/ spreading into space	Metal oxidation	Electrostatic interactions
		Enhanced reactions

a

Placement of many processes under the diverse categories are tentative, as the details of grain growth cannot be ascertained past experiments that reasonably gauge the atmospheric condition.

b

Previous generations of stars lose mass in various ways every bit they age and change. Isotopically distinct material (presolar grains) are identified in meteorites.

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Thermo-chemical evolution of the Earth

Anne M. Hofmeister , in Heat Send and Energetics of the Earth and Rocky Planets, 2020

ix.3.1.3 Oxidation during assembly releases gas

The main ices in nebulae, H ₂O, CO_two, and CO, freeze at 273, 195, and 68   K, respectively. Although the starting conditions are ~8   K, collisions of grit in the winds of accession should create heat and dislodge water ice. Considering CO is abundant, this ice would nucleate on Fe⁰, where collisions of this circuitous with copious H_ii gas permit a reaction:

(nine.iii) $\left\{{\mathrm{Fe}}^0+\text{CO}\right\}+{\mathrm{H}}_2\to \text{FeC}+{\mathrm{H}}_{\mathrm{two}}\mathrm{O}$

Both Ni and Fe metal are known catalysts (e.g., the Haber process). The reaction of Eq. (nine.3) helps to explicate H₂O being more abundant than CO_ii upon densification of nebula into clouds and comets (Fig. nine.ii), and the ubiquitous presence of graphite and carbides in iron meteorites. Chapter eleven provides farther word. Low temperatures limit reaction rates, so the earliest stages (i.eastward., product of the core and the lower pall) should involve mostly dust collection and compaction.

As temperatures climb, perhaps due to Solar ignition or radiative transfer decreasing in the densifying nebula, CO is the first species to vaporize, whereas highly polar and reactive h2o is retained, as observed for comets (Fig. ix.2). Considering carbonic acid is effective at rusting atomic number 26 and rusting is exothermic (meet websites) and therefore highly favored, some other reaction may oxidize atomic number 26 as accretion gain:

(9.4) ${\mathrm{Iron}}^0+{\mathrm{H}}_2\mathrm{O}\to \text{FeO}+{\mathrm{H}}_2{(\text{promoted}\text{by}\text{dissolved}\text{CO}}_2)$

(9.five) $\text{FeO}+{\text{MgSiO}}_3\left(\text{enstatite}\right)\to {(\text{Mg},\text{Iron})\text{SiO}}_4\left(\text{Fe-rich}\text{olivine}\right)$

(ix.6) ${\mathrm{Mg}}_2{\mathrm{SiO}}_{\mathrm{four}}{\left(\text{forsterite}\right)+(\text{Mg},\text{Fe})\text{SiO}}_4\to 2{(\text{Mg}}_{1.5}{\mathrm{Fe}}_{0.5}{\mathrm{SiO}}_4\left)\right(\text{meteoritic olivine})$

These reactions are simplifications of a very complex electrolytic corrosion process that releases hydrogen gas (see websites; Graedel and Frankenthal, 1990). Hydrogen gas is light and diffuses upward, which is promoted by compaction as material is added. This loss occurs at shallow levels, as a gas-producing reaction is not promoted by pressure. Departing H₂ gas carries some heat of reaction away. Substantial heating will vaporize CO₂ ice, which volition ascent through the porous substrate. This process is consistent with H₂O dominating comets. The presence of acidifying H₂Southward, And so, and And so_two, every bit occurs in comets (Fig. 9.2; see also Fig. 10.6), promote electrolytic corrosion (Graedel and Frankenthal, 1990).

Our proposal is supported by compositional data. As shown in Tabular array ix.four, the different types (H, 50, LL) are associated with different ranges of Fe/Mg in the olivine. From Table ix.2, an boilerplate ordinary chondrite has about ½ of the iron alloy of an enstatite chondrite, which on average are composed of 57   mol% MgSiO₃ +33   mol% Fe +10   mol% FeS. Reacting near ½ of the metal in an enstatite chondrite via Eqs. (ix.three) and (9.4) yields Fo77, which is close to the boilerplate of ~Fo79 for the ordinary chondrites, by number proportions. Rusting much iron metal in an enstatite chondrite makes 50 and LL chondrites, whereas rusting less metal makes type H. These reactions are consistent with the observed proportions of rock to metal in ordinary chondrites (run across Lodders and Fegley, 1998, their table 16.11, and Ringwood, 1961).

Also, the proportion of forsterite to enstatite in the precursor material affects the amount of Atomic number 26²⁺ in the olivine of the ordinary chondrites. Many reactions are possible, since rusting really puts Fe²⁺ ions into solution, which tin ion exchange with Mg in forsterite and other minerals. Analogous reactions involving troilite could also occur, and would similarly evolve gas, while oxidizing atomic number 26 and contributing to the variability. Loss of H₂ gas and volatilizing ices, which depart as vapor, mitigates heating of the surface.

Rusting of the surface of planets occurred equally these formed. Nevertheless, the current upper mantle is not as iron rich as an average ordinary chondrite: the composition at least for the upper 250   km is Fo90, rather than Fo79. The correlation of younger ages with higher Fe contents suggests that an unstable density profile was formed, and furthermore that the more magnesian olivines probable formed early, as the transition zone accumulated, merely were buried, run across Section 9.iii.2.

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Volatiles: Origin and Transport

Jennifer A. Grier , Andrew S. Rivkin , in Airless Bodies of the Inner Solar System, 2019

The Accretion and Development of Endogenic Volatiles

Volatiles were arable in the nebula where our Solar System formed, if evidenced only by the gas and ice giants nosotros accept in our organisation today. But most of the volatile elements (and subsequent compounds) are also cosmically highly abundant, so their incorporation into the solar nebula is no surprise.

Table ten.i lists the 10 most arable elements in the solar system, with their abundances relative to silicon listed. The Large Bang produced the get-go elements, including a massive amount of H, lesser corporeality of He, and small amounts of B and Li. After, these materials were gravitationally gathered into stars, which produced all heavier elements through stellar fusion, supernova explosions, black hole interactions, etc. We see from the table that the elements and constituents of volatile molecules are highly arable, and expected to dominate in protostellar nebulae. Of note are the high abundances of both hydrogen and oxygen, making the mutual presence of water in the solar system, in locations where ice is stable, no surprise. Carbon, nitrogen, and sulfur compounds would also be expected from these abundances, particularly in compounds with hydrogen and/or oxygen.

Table 10.1. The 10 most arable elements in the solar system, relative to silicon

Chemical element	Ab. Rel Si
Hydrogen	28,000
Helium	2700
Oxygen	24
Carbon	10.0
Nitrogen	three.1
Neon	3.0
Magnesium	1.one
Silicon	1
Iron	0.ix
Sulfur	0.52

The most common elements are not typically found in rocks, other than oxygen, one of the building blocks of silicate minerals. The most common elements are found in volatile ices similar water water ice, carbon monoxide and dioxide, ammonia, marsh gas, etc.

Nosotros wait volatiles to exist less abundant closer to the furnace that is our Sun, since these would be unstable or driven off by evaporation, vaporization, etc. However, we think fifty-fifty the inner planets, including the airless bodies of the inner solar system, had some corporeality of volatiles available when they formed.

The "snow line" (also "ice line") is the distance from the Sun at which the conditions in the solar nebula would take allowed water ice to condense. The concept of a snowfall line can be applied to whatever of the substances we discuss here—the "carbon dioxide snow line" was always at a further altitude than the ice line, and the "nitrogen snow line" farther still. We see abundant amounts of water and other volatiles beyond the ice line (at almost the current distance of Jupiter) and we run across much less inside that line (the rocky inner planets) (Fig. 10.2). However, once ice grains condensed beyond the snow line, they may accept migrated inwards toward the Lord's day through gas drag (Cyr et al., 1998). In addition, new dynamical models (such as the Nice Model and G Tack: O'Brien et al., 2014; Morbidelli et al., 2015) suggest a great bargain more mixing throughout the solar system, on both small and large scales, than was previously considered, an idea supported past the range of materials returned by the Stardust comet coma sample render mission. As noted in Chapter iii, these models suggest that volatiles, especially ice, may have been delivered to the inner solar arrangement by water-rich planetesimals early in solar system history. These models farther advise that the majority of mass in today's asteroid chugalug may have formed in the giant planet region (Walsh et al., 2012).

Fig. 10.ii. Solar system germination models take been created to calculate the abundance of different volatiles at different locations and times in the solar nebula. As can exist seen here, the different "ice lines" motion and the shape of the affluence curves change—since the amount of nitrogen, for instance, is fixed, the ability of Northward₂ to newly accrete in a location affects the amount of NO and NH_iii there, which in turn frees O and H to react with each other or carbon. These models also find that condensation occurs over a range of distances and isn't a elementary footstep function.

(From Dodson-Robinson, South.Due east., Willacy, K., Bodenheimer, P., Turner, N.J., Beichman, C.A., 2009. Water ice lines, planetesimal limerick and solid surface density in the solar nebula. Icarus 200(2), 672–693.)

The Earth, although inside of the snow line, stands as an example of a h2o-rich rocky world. Some bodies like the Moon suffered some course of major result or procedure (differentiation, behemothic impact, etc.) that collection many of the original volatiles out of the planet, leaving backside what were long considered to exist utterly "dry" worlds. (See Chapter iii for give-and-take of discovery of water on the Moon, and the post-obit paragraphs for give-and-take of lunar volatiles.) Our image has shifted. Now we know that even the "dry" airless bodies can accept ample volatiles incorporated in their surfaces in places, and that such substances play a key role in the evolution of those surfaces.

Those volatiles original to the body that withal remain can exist liberated from deep inside by impacts and past volcanic activity. Impacts are happening all the time on airless bodies (Affiliate 7), so this process is always occurring and available to bring volatiles up from the depths, as well as to, conversely, degas surface volatiles. For many bodies, any meaningful volcanic activity happened billions of years in the past. Simply even on the Moon there is some proposition that may all the same be plenty volcanic activity to locally degas volatiles. Some features on the Moon accept exceptionally depression crater densities, and odd morphologies. Such areas might be the sites of very recent (~   10   Ma age) volatile degassing. If this is the case, so it is possible that some of the volatiles currently detected on the Moon derive from contempo events. Areas where this may be happening on the Moon, sites of "lunar transient events" are discussed in Chapter 11.

Finally, some asteroids accreted pregnant amounts of volatiles and maintain them to this solar day. For the virtually part, meteoritic hydrated minerals are confined to the C chondrites, though there are some exceptions mentioned hither. The CM and CI groups of carbonaceous chondrite meteorites have abundant phyllosilicate minerals, which have OH every bit function of their structure, also as other hydroxide minerals (Rubin, 1997). As a upshot, 10% or more of their mass tin can be h2o or hydroxyl. It is thought the CM and CI parent bodies formed as mixtures of ice and anhydrous silicates that were then heated beyond the melting point of water ice, assuasive aqueous alteration reactions to begin and hydrated minerals to form (Brearley, 2006). Many of these reactions are exothermic, so once they begin additional estrus is generated to melt more ice and bulldoze farther reactions, creating a self-sustaining process until all the ice is melted. Depending on the specific situation, continued heating could occur, metamorphosing and somewhen dehydrating and destroying the hydrated minerals. It is thought this happened to some carbonaceous chondrites (Nakamura, 2005). On the other hand, objects that never experienced sufficient heat to melt ice in the beginning place may retain their original ice in their subsurface and interiors. At least some of the so-chosen principal chugalug comets or active asteroids appear to have activity driven past sublimation of near-surface ice, consistent with existence such never-heated objects. Given that the CM and CI parent bodies tin have ~   x% of their mass in water, it is reasonable to expect any unreacted bodies to also take ten% of their mass in water ice, if not more. Examples of all of these types of objects, from the parent bodies of hydrated and metamorphosed carbonaceous chondrites to potentially unreacted physical mixtures of water ice and anhydrous minerals, may be present in the C-complex asteroid population, equally further discussed here.

Models of the thermal evolution of Ceres past McCord and Sotin (2005) and Castillo-Rogez and McCord (2010) prior to Dawn'southward inflow predicted that body to exist differentiated, with an icy mantle higher up a rocky core. With the benefit of data from Dawn, it is now thought that Ceres is only partially differentiated, with an ice-rich only non pure ice interior (Park et al., 2016). Whether this interior structure reflects a loss of ice early in Ceres' history (Castillo-Rogez et al., 2016) or reflects a largely undisturbed state is a subject of ongoing research. While Ceres is the but object of its kind that we have visited with spacecraft, there is the prospect that other large asteroids have had like histories and now accept similar interior structures (Schmidt and Castillo-Rogez, 2012).

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Volume 3

Yard.J.H. McCall , in Encyclopedia of Geology (2nd Edition), 2005

The Origin of the Earth

The Earth was formed from the nebula that produced the Solar Organisation. It is almost universally accustomed that the Dominicus, the planets and their satellites, the asteroids, and the comets of the Oort 'cloud' grew from a deject of gas and dust that contracted under its ain gravity. The cloud of gas and dust had some degree of rotation (Effigy 12) and, as the eye contracted, the athwart momentum forced the remains into a flattened disk, in the feather perpendicular to the centrality of rotation of the proto-Sunday. This was manifestly a very rapid process, perhaps taking place over 10 000 years. The dust particles accreted heterogeneously to produce lumps that formed planetesmals, the first small solid bodies, and cooling occurred.

Figure 12. The development of the Earth equally a planet in the Solar Arrangement from the nebula.

We tin judge the composition of the original nebula from the solar abundances of elements, obtained spectrographically, and analysis of archaic meteorites (e.g. carbonaceous chondrites); however, these are merely estimates, and a second method is based on an assumption that may not exist entirely accurate. Radioactivity-based dating methods (relying on the decay of radiogenic isotopes), every bit applied to terrestrial igneous rocks, allow us to date the condensation to 4500   Ma agone (the and so-called 'historic period of the Globe'), and it is believed that condensation was rapid, taking well-nigh 100 000 years.

Accretion of the World and the other planets may take been slow and heterogeneous or rapid and homogeneous. Which of these two models is right remains uncertain, but the offset model, if correct, could have produced a layered mantle.

After condensation and accretion, the World evolved rapidly into a planet with most of its present properties. This could have taken no more than 500   Ma and was probably effected more rapidly: the oldest known minerals are zoned zircons from the Mount Narryer gneiss terrain, Western Commonwealth of australia, which requite spot SHRIMP ages of 4400   Ma and are believed to take been formed by partial melting of preexisting granitic crust. This evidence indicates that in that location was solid rock at the World's surface astonishingly soon after condensation and accretion.

The oldest actual rock dated is a component of the Acasta Gneiss in the Slave Province of Canada, which has an age of 4000   Ma. Despite the existence of the Mount Narryer minerals, there is no other really pregnant evidence from geology of the World pertaining to the 500   Ma prior to the formation of the Acasta Gneiss, and this is called the 'Hadean Eon', geology's dark ages. Nosotros do know something of this period from the Moon – the lunar rocks brought dorsum by Apollo and Lunik date from this period – and it is reasonable to invoke a bombardment of the Earth, like the Moon, during this period. Nonetheless, in that location is no trace of this in the geological record of the Earth. The lack of bear witness of ejecta from the Moon in Space means that it is difficult to attribute all the heavy early cratering of the Moon to battery by impacting objects from Infinite. Nevertheless, the early Earth must have swept upward asteroids, comets, and whatever other droppings in its path for a few 1000000 years after information technology attained its present size.

The Globe, after accretion, must have heated up internally and melted in lodge to split, gravitationally, the core (part liquid and part solid), mantle, and crust. This heating would have been caused by the radioactivity of elements and by gravitational energy – the latter as the dust, or dust and planetesmals, condensed and accreted into a ball. It has also been suggested that heat could take been produced past impacts on the surface, though much of this estrus would have dissipated into Space. The heavy elements, mainly atomic number 26, would have gravitated to the core, and the lighter silicate fabric would have ended upwards in the mantle and crust. A primitive and highly mobile surface layer, peradventure a magma ocean, would have initiated the crust, and from this minor-scale proto-continents separated. More 'style-out' models for the formation of the crust invoke early on impacts, the subsidence of big impact-generated basins in the basaltic proto-chaff, and partial melting of the basalt to grade the proto-continents. The early protocontinents were certainly small, and the extent of the continents increased until around 3200   Ma, when the Kaapvaal (S Africa) and Pilbara (Western Australia) cratons were formed.

The very lightest volatile elements would have separated at the same fourth dimension to class the proto-ocean and proto-atmosphere. This volatile component would have had a majority limerick of ten–20% water and contents of other volatiles in line with those of carbonaceous chondrite meteorites, according to the chondritic Earth model. The start atmosphere was near certainly reducing rather than oxygenating. Our present oxygen-containing temper was derived later, though how much after is debatable, and relates, in its origin, to the onset of biological activity. How and when life originated is still a mystery – there are models that suggest that it originated within the early on developing Earth, simply there are also models that suggest that it arrived on the planet from comets or archaic meteorites.

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Electron Kappa Distributions in Astrophysical Nebulae

D.C. Nicholls , ... Fifty.J. Kewley , in Kappa Distributions, 2017

17.12 Concluding Remarks

If we adopt kappa distributions for the nebulae electron energies, we tin can resolve a long-continuing problem where measurements of electron temperatures and chemic abundances using different methods yield discrepant results. While at that place are alternative explanations of the abundance discrepancies, there are concerns that the idea runs counter to what they encounter as the findings of such regime. It is likely that the true explanation of abundance measurement differences will have a multifariousness of causes. Nonetheless, kappa electron energy distributions are undoubtedly nowadays in HII regions and PNe and contribute to the abundance and temperature discrepancies. Electron temperature errors consistent from ignoring this possibility may corporeality to thousand or 2000 K. This is a new field of research, and we need to undertake observations to look for unambiguous signatures of the kappa distribution to measure kappa and explore its physical effects.

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Physical constraints on the initial conditions and early on evolution of the solar system

Anne Chiliad. Hofmeister , Robert E. Criss , in Heat Transport and Energetics of the Earth and Rocky Planets, 2020

4.4.2.1 Impacts on formed bodies: how much oestrus and where?

After the initial collapse and wrinkle of the nebula to form a proto-Sun and its proto-planets in a deejay-like configuration, the formed or forming Sun acted externally to the planets (Fig. 4.9). Due to the immense mass of the growing Sun, any fabric that was not in a stable orbit was drawn towards the center. Thus, the orbiting planets intercepted late arriving material fatigued from smashing distances as it moved in, much similar a machine windshield impacts flying insects during a road trip. Nearly planetary impacts were pocket-size and surficial so any heat released was chop-chop radiated to space, then that energy of standoff is of no outcome to the thermal evolution of the planet. Similar the motorcar, small impactors do fiddling damage, merely harm and potential heating increase with impactor size.

Figure 4.9. Sketch of the late phase bombardment in the inner Solar system. Perspective is crude, and bodies are not to scale. The debris would take been drawn towards the Sun in the central plane as indicated, simply likewise in 3-dimensions from the nebula. Thin arrows point acceleration of the debris towards the Sun. The lunar orbit (dashed ellipse) presents a much larger cantankerous section to the incoming debris than the expanse of the Earth.

Observed heavily cratered surfaces have dominated thinking regarding formation of the Solar System for 50 years (due east.chiliad., Armitage, 2011). This late stage is of import for adding material, and may have given the upper drapery and Moon the loftier potassium and oxygen-18 concentrations they share with outer Solar system material (Chapter one). Mercury, being close to the Sun received an backlog of impacts, ejecting drapery and cadre cloth, some of which was shepherded into the asteroid chugalug, providing the HEDs course (Hofmeister and Criss, 2012b). Evidence is provided by Mercury's core being overly big compared to its mantle and its obit being eccentric and tilted from the shared airplane of the other planets. In add-on to the planet nearest the Sun being dilapidated, Mercury and Venus lack moons, while World has so far been able to retain its big Moon due to its greater distance from the Dominicus. Lunar recession signifies that the Sunday is slowly capturing the Moon (Chapter 5).

Impacts both add textile and squirt fabric, as is well known, and have provided meteorites from Mars and the Moon, and probably from Mercury. The net gain, net loss, or mass-neutral exchange depends on whether the receiving object is well consolidated material or a conglomerate, like undifferentiated meteorites, plus the speed and size of the impactor. Fig. four.v suggests that the Moon accreted from the grit cloud surrounding the Early on Earth in the aforementioned way every bit the satellite systems accreted from the grit and gas clouds surrounding the giant planets. What is unusual is the Moon's big size relative to its companion planet, which suggests either that either material was added during the belatedly stage, or that the Moon has recessed further than suggested by its current outward velocity. Lunar recession is accelerating with time, a consequence of the Dominicus applying torque to the Moon during its non-planar orbit (Hofmeister et al., in prep.; Chapter 5), and thus the former can be ruled out. Fabric apparently was added to the Moon, and furthermore was also added to the Earth, as gauged from comparison to Venus (Figs. 4.3 and 4.4). Progressing toward the Sun, credible changes in the mass inventory during the final stage are:

(4.14) $\text{Globe}(\text{proceeds})\to \text{Venus}(\text{neutral})\to \text{Mercury}(\text{loss})$

This sequence is consequent with the focus of impactors on the Dominicus and their acceleration to high velocities toward the center.

Was oestrus added forth with the tardily arriving material? This is almost certainly the case, merely the Moon'due south surface indicates shallow placement of both. Equivalent behavior is expected for the Moon and Earth because the impactors are being drawn to the Sun, not to the planets, which simply occasionally block and intercept their path. Whatsoever primordial oestrus so supplied to the planets merely warmed their outer layers.

In contrast, the loss of spin and the associated heat production early (Table 1.three) is quite large. The exponential decay suggested in Fig. 1.11 will be used in Chapter ten to gauge rut production in the early World.

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Meteorites, Comets, and Planets

J.E. Chambers , in Treatise on Geochemistry, 2007

one.17.6.3 Disk Instability

GI can act on the gaseous portion of the nebula in improver to solid textile. A portion of a massive gas disk can spontaneously collapse due to its own gravity if the random motion of the gas (and hence the temperature) is depression enough. The unstable region forms a dense clump that is likely to have a mass comparable to Jupiter. In the "disk instability" model, the giant planets formed in a unmarried-footstep process in a gravitationally unstable disk. Whether dense clumps become on to course planets depends sensitively on how fast the clumps cool (Cai et al., 2006). Some calculations advise that cooling rates would have been fast enough for gravitationally bound objects to form (Boss, 2000). Numerical simulations bespeak that these objects would continue to survive and abound denser over time for at to the lowest degree several hundred years (Mayer et al., 2002; Boss, 2005). It is possible that solid materials would have sedimented to the center of a clump to grade a core, of the kind thought to exist at the centers of Jupiter and Saturn. Nonetheless, this possibility has non been explored in detail to engagement.

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Volume ane

Véronique Vuitton , in Encyclopedia of Geology (2d Edition), 2021

Interior

Titan is believed to have formed in Saturn's circumplanetary nebula. The accreted material formed the interior of Titan, whose limerick is supposed to be a mixture of rock, ice and volatile compounds. The surface of Titan is covered with a layer of ice of a thickness of less than 30–40   km on average merely that could be inhomogeneous in thickness in places (Béghin et al., 2012). This layer was deduced from gravity field data by radar sounding and from conductivity measurements of the Huygens Atmospheric Structure Instrument (HASI). Under the ice crust, an underground ocean of liquid water could be enriched with mineral salts or ammonia. There are ii hypotheses for the layering structure below this clandestine ocean: either (i) a layer of pure ice, followed past a layer of a mixture of ice and stone overlying a cadre of pure (mainly anhydrous) rock; or (two) a hydrated layer followed by a medium density rock core, composed mainly of hydrated silicate minerals. Both of these scenarios are consistent with the deduced normalized moment of inertia, which is between 0.33 and 0.34. This indicates that Titan is non fully differentiated and does not have a big iron core (Fortes, 2012). Titan'due south physical data are given in Tabular array 2.

Table two. Titan's physical characteristics.

Hateful radius	2574.73   ±   0.09   km (0.404 Earths)
Mass	(1.3452   ±   0.0002)   ×   1023  kg (0.0225 Earths)
Mean density	1.8798   ±   0.0044   g.cm−   three
Surface gravity	1.352   m   s−   ii (0.138   yard)

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Planets, Asteriods, Comets and The Solar Organisation

A.P. Boss , F.J. Ciesla , in Treatise on Geochemistry (Second Edition), 2014

2.iii.3.two.1.2 Ionization structure and layered accession

MRI is a powerful miracle simply is limited to affecting nebula regions where in that location is sufficient ionization for the magnetic field, which is coupled but to the ions, to accept an effect on the neutral atoms and molecules. The MRI studies described earlier all presume platonic MHD, that is, a fully ionized plasma, where the magnetic field is frozen into the fluid. At the midplane of the solar nebula, all the same, the fractional ionization is expected to be quite depression in the planetary region. Ambipolar improvidence, resistivity (ohmic dissipation), and Hall currents are all constructive at limiting magnetic field strengths and suppressing MRI-driven turbulence (Desch, 2004), merely a partial ionization of only ~1 ion per g billion atoms is sufficient for MRI to proceed in spite of ambipolar diffusion and ohmic dissipation. Shut to the protosun, deejay temperatures are certainly high enough for thermal ionization to create an ionization fraction greater than this and thus to maintain total-diddled MRI turbulence. Given that a temperature of at least 1400   K is necessary, MRI instability may be limited to the innermost 0.2   AU or and then in quiescent phases, or as far out every bit ~1   AU during rapid mass accretion phases (Boss, 1998; Stone et al., 2000). Once started, MRI may be able to drive sufficient ionization in the disk to maintain itself (Inutsuka and Sano, 2005) at a level determined by magnetic field reconnection events (Sano et al., 2004).

At greater distances, deejay temperatures are besides low for thermal ionization to be effective. Cosmic rays were thought to exist able to ionize the outer regions of the nebula, but the fact that bipolar outflows are likely to exist magnetically driven ways that catholic rays may have a hard fourth dimension reaching the deejay midplane (Dolginov and Stepinski, 1994). However, the coronae of immature stars are known to be prolific emitters of hard ten-rays, which can penetrate the bipolar outflow and accomplish the disk surface at distances of ~1   AU or so, where they are attenuated (Glassgold et al., 1997). As a upshot, the solar nebula is likely to exist a layered accretion disk (Gammie, 1996), where MRI turbulence results in inward mass transport inside sparse, lightly ionized surface layers, while the layers below the surface, around the deejay midplane, practise non participate in MRI-driven transport. Thus, much of the disk, from but below the surface to the midplane, is expected to be a magnetically expressionless zone with much lower levels of stirring than the active surface layers. Dust grains are highly efficient at sweeping up free electrons, reducing the fractional ionization of the gas and thus increasing the extent of the expressionless zone (Ilgner and Nelson, 2006). Layered accretion is thought to be capable of driving mass inflow at a rate of about 1   M_⊙ per 100   Ma, sufficient to account for observed mass accretion rates in quiescent T Tauri stars.

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Cosmology

John J. Dykla , in Encyclopedia of Concrete Science and Engineering (3rd Edition), 2003

Two.D Viewing the Island Universe

Some hazy patches amongst the fixed stars, such equally the Andromeda nebula or the Magellanic Clouds, are clearly visible to the unaided homo eye. When Galileo observed the band called the Milky way through his telescope and was surprised to observe it resolved into a huge number of faint stars, he concluded that most nebulosities were composed of stars. Early in the eighteenth century, the Swedish philosopher Emanuel Swedenborg described the Galaxy every bit a rotating spherical assembly of stars and suggested that the universe was filled with such spheres. The English language mathematician and instrument maker Thomas Wright also considered the Milky way to be one among many only supposed its shape to be a vast deejay containing concentric rings of stars. Past 1855, Immanuel Kant had further developed the disk model of the Milky Mode by applying Newtonian mechanics, explaining its shape through rotation. He causeless that the nebulae are similar "isle universes."

Early in the nineteenth century, however, William Herschel discovered the planetary nebulae, stars in association with true nebulosities. This reinforced a possible interpretation of nebulae every bit planetary systems in the making, in agreement with the theory of the origin of the solar organization developed by Pierre Simon de Laplace. John Herschel'southward observations of 2500 additional nebulae, published in 1847, emphasized that they were generally distributed away from the galactic plane. The "zone of avoidance" was used past some to argue for the physical association of the nebulae with the Milky Manner. (It is now known that dust and gas in the plane of the galaxy diminish the intensity of calorie-free passing through it, whether from sources within it or beyond.) In 1864, Huggins studied the Orion nebula and found information technology to brandish a bright line spectrum like to that of a hot gas. Also, photographs of Orion and the Crab nebula did not bear witness resolution into private stars. As the twentieth century began, the question of the nebulae was intricately linked with that of the construction of the Milky Way. The size of nebulae was still quite uncertain in the absence of any reliable distance indicators, only most astronomers believed the bear witness favored because the nebulae office of the Galaxy, thus reducing the universe to this one island of stars.

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